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Danger for hot air balloon if someone jumps out
I recently saw a video that I believe was filmed in Turkey where a skydiver was prevented from jumping out of a hot air balloon by the operator/pilot.
Man tries to jump out of a hot air balloon (YouTube video)
His rationale for stopping the jumper was:

the balloon could go out of control and fly up unexpectedly
he could lose his licensing.

He goes into a long explanation of the dangers of someone unexpectedly jumping out but something seems off to me. I have done balloon jumps and the basket is incredibly stable when people jump. The basket and people must weight a couple thousand pounds so I can't see how a person with a parachute could have the dramatic effect that is described in his explanation. I am in no way arguing that he should have been allowed to jump, just saying I am not buying the potential danger described.
Does the description of the danger hold any air?

Re "His rationale for stopping the jumper was 1) the balloon could go out of control and fly up unexpectedly, 2) he could lose his licensing."-- I haven't watched the video, but it seems from this description that there should have been more of a pre-flight discussion.  What, was the existence of the parachute and the intention to jump not revealed until the flight was already underway?  ??!!??

After watching the video-- perhaps he had the chute hidden under a jacket or coat that he removed in the middle of a flight?  (Edit) sounds like they were at around 3000' AGL when this all happened--

In the UK, the ANO says "the dropping of persons by parachute and which is made under and in accordance with the terms of a parachuting permission granted by the CAA under article 90"  so you'd need a permit for a parachute jump.  Turkey is a member of the ICAO so I'd guess something similar applies there.

The balloon pilot should have just yanked the wannabe skydiver's D-ring so the chute would fall out in a clump on the floor of the balloon basket!  (I guess the springy thing in the pilot chute could have been a problem though  :(  )   Maybe in the future all balloon pilots should carry a bicycle cable lock to lock the skydiving rig to the balloon basket; that would be a way to instantly end the issue --

@quietflyer That adds much more danger than it takes away. If he was able to get a chute out while in the basket there is no telling what would happen. If he was able to pull his pilot chute all the way out, the main parachute would probably just fall out in the deployment bag. If he pulled the reserve handle, it could get ugly as that is spring loaded and could potentially inflate. For the most part D-rings are going the way of the dinosaur being replaced by padded handles.

@DaveGremlin “because it’s illegal” is an obvious answer and I do sympathize with the balloon operator who could get into legal trouble but the safety claims seem like a stretch…

@JonathanReez Can you please state what you expect from a new answer? I mean, what part do you think quite flyer's answer doesn't address? Please state exactly what kind of question you want to be addressed. And can you please tell us what do you mean by "canonical answer"?

@AircraftEnthusiast007 ideally I want to see a calculation of whether the air balloon in question was at risk or not, along with examples of balloons crashing to the ground in similar situations. I’m only interested in the physics aspect, not the legal side of things.

@JonathanReez OK, got it. However, it seems like there's no great deal of actual danger in this, just that it is not recommended and illegal.

@AircraftEnthusiast007 why is it "not recommended" though?

@JonathanReez Well, if the balloon only have two passengers, and one of them jumps out then it is quite dangerous, but it can be compensated by reducing gas to the burner. But in the video OP provided, there seems to be like at least 4 or more people onboard, so there's not that much danger.

Apparently, faillure of the parachute valve is called parachute stall, and it can lead to deadly outcomes.

@JonathanReez I'm researching on this topic, and I would like to know what you mean by "calculation". By "calculation", do you mean you want a pure physics and mathematical proof of why and how this is/is not dangerous?

@AircraftEnthusiast007 yes, some math would be good

@JonathanReez OK, but we don't know the exact altitude, density of air, weight of basket, and air temperature on that day, so how should we prove it? I mean, calculating lift requires many variables that are not given in this case.

From the balloon's perspective, the jumper is just ballast.

The basket and people must weight a couple thousand pounds so I can't
see how a person with a parachute could have the dramatic effect that
is described in his explanation. I am in no way arguing that he should
have been allowed to jump, just saying I am not buying the potential
danger described.
Does the description of the danger hold any air?

The explanation starting at 2:30 in the video (

 ) is enlightening.  By "parachute" he is referring to the round top of the balloon which is only held in place by some velcro, plus the upward pressure of the hot air which forces the "parachute" upwards against the load tapes.  Normally the "parachute" is only pulled slightly downward occasionally on one side to spill a small amount of hot air, until after touchdown when it is pulled completely down to deflate the envelope as soon as possible to prevent the wind from dragging the balloon along on the ground.  A downward aerodynamic load on the top of the balloon due to an abnormally high ascent rate could dislodge the "parachute" entirely and let all the hot air out.  This is why skydivers should normally only be allowed to exit when the balloon has zero vertical speed, or is in a descent.  Likewise for hang glider drops.
The "parachute" or "parachute valve" can be seen on a diagram on this web page: https://hotairflight.com/blog/parts-of-a-hot-air-balloon?expand_article=1
This link  some interesting reading about skydiving from balloons.

Last link is really more about logistics than flight dynamics/safety but still is an interesting (and kind of funny) read-- some more links could surely be found that more directly address the flight dynamics/safety issues--

So if: 1) The balloon is moving upwards. 2) The person jumping is a large percentage of the total weight to have maximum effect(small balloon and basket with few passengers etc.) -- a danger exists. I believe the pilot in the video may have been overstating the danger but he probably did that to scare off any additional morons that would want to try this without permission.

TLDR: Running the numbers with some assumptions, an average person in average conditions cannot induce a parachute stall by jumping out of the basket
I'm no ballooning expert, but I can give some reasoning. This relies heavily on the AAIB report found here
Background information on how balloons can crash
Normally, balloons have a parachute at the top of the balloon. Normal balloons are designed such that once you open the parachute, the flow of air that goes along it, generates lift, which pushes up the parachute and closes off the gap. This means it's a self-restoring element. However, this only works if the parachute can 'use' the airflow to generate enough lift to push itself back up. Like a normal wing, the parachute can end up in a condition where the flow stalls and lift is greatly reduced, meaning, the parachute cannot return to its closed position.
This is called parachute stall . Similar to wings, a stall is dependent on the exact flow conditions and the geometry, so I cannot say in a generic way when it will happen.

Source
Analysis on what can factors influence parachute stall
However, we can look at what's influencing the forces on the parachute.
Let's look at a parachute in normal conditions. It looks like this:

If we look at the balance of the forces in z on the parachute:
$$ p_{air} \cdot A + W_{balloon} = p_{balloon} \cdot A \tag{1}\label{1}$$
or reorganized:
$$ W_{balloon} = \left(p_{balloon} - p_{air}\right) \cdot A \tag{2}\label{2}$$
If we want to let some air out, we can pull the rope, which will reduce $A$, and reduce the net upward forces due to the pressure difference, causing the parachute to fall.
However, the force balance of Formula \eqref{1} only holds when we're not moving vertically. If we go up at a certain speed, the top of the parachute will collide with the air, causing an increase in pressure, called dynamic pressure:

The dynamic pressure is added to the force balance of \eqref{1}:
$$ p_{air} \cdot A + W_{balloon} + p_{dyn} \cdot {A} = p_{balloon} \cdot A \tag{3}\label{3}$$
The dynamic pressure is related to the flow velocity V:
$$ p_{air} \cdot A + W_{balloon} + \frac{1}{2}\rho V^2 \cdot {A} = p_{balloon} \cdot A \tag{4}\label{4}$$
So, rising (by the pilot, or by a passenger jumping out), can cause the parachute to fall, and then it's dependent on the exact conditions if a stall occurs or not. If a stall occurs, the parachute will not return to its closed position, and the balloon will let out hot air and crash.
This is confirmed by AAIB:

High rates of climb: With an increase in the rate of climb comes a corresponding
increase in the pressure on top of the balloon envelope and above the parachute.
The faster the climb rate, the higher the pressure. As this pressure increases,
the difference between the pressure above and below the parachute valve
decreases, resulting in a decrease in the distance the parachute can be pulled
into the envelope before it reaches the stall point. Aggressive venting whilst in
a fast climb could more easily result in a parachute stall.

Of course, with the pilot unaware, you could have a combination of the two, where the pilot is making the balloon rise quickly and then somebody jumps out, which would exacerbate the risks.
Given Formula \eqref{4},the right input data and an estimation of how much $\Delta V$ one person jumping would generate one could estimate the risk of parachute stall.
However, without knowing more about the exact flow conditions and geometry of the parachute, we cannot determine if it's stalled.
Estimating risk due to person jumping
What we can do, is to look at the vertical speeds that have led to parachute stall, and then see if a person jumping out could induce those velocities.
From the document we find the vertical velocity where a parachute stall happened:

The AAIB obtained flight data from one case involving a lightly loaded balloon that climbed
from a low height at 8 m/s, a similar rate to G-CMFS’s last climb before the accident. The
pilot described a parachute stall which caused the throat to close. The balloon stopped
climbing and began descending at about 6 m/s (1,200 ft/min). He managed to burn through
the envelope to re-inflate it and landed shortly afterwards.

Can a person jumping out of the balloon lead to such a vertical velocity?
This source gives the following mass for a hot air balloon with 5 passengers: 800 kg, with a passenger weight of 80 kg each.
Once the passenger jumps, there is an upward imbalance of 80 * 9.81 = 784.8 N. But what is the 'terminal velocity' achievable by this imbalance?
We can set the upward force equal to the drag.
$$ F_{up} = \frac{1}{2} \cdot \rho \cdot V^2 \cdot C_D \cdot A $$
$C_D \cdot A$ is 254m2 for a typical hot air balloon in ascending flight.
At a typical flight altitude of 3,000 ft, the density is 1.12 kg/m3
$$ 784.8 = \frac{1}{2} \cdot 1.12 \cdot V^2 \cdot 254 $$
Solving for V gives us an upward terminal velocity of 2.34 m/s; less than the mentioned 8 m/s needed for parachute stall.
So the average passenger of the average hot air balloon in average conditions, with a non-responding balloon pilot, can not make it crash by jumping out.

For my own curiosity of how much mass lost would give the balloon 8 m/s terminal velocity, (1/2) * 1.12 * 8 ^2 * 229 -> 8207N -> 1,845lbs

@mack, I guess I forgot the square root. I did it on my phone :) Will edit

Yes, the danger is real. A hot air balloon basket weighs very little, and can often be easily carried by two people. I haven't seen your video but the total payload can be less then 750 pounds so having two hundred pounds of ballast jump out can make a real difference, causing the balloon to gain a lot of height and potentially putting it into very different wind conditions.
At the end of the day, everything that happens is the pilots responsibility. If they are unsure of any aspect of the flight, they must take the least risky option.

This goes against my experience as stated in my question.

Even if the payload plus basket weighs only 750 lb, the entire aircraft must weigh roughly double that to stay buoyant (it must displace 750 lb of atmosphere, with something not much lighter).  200 / 1500 isn't quite so dramatic.

Plus the weight of the fabric "envelope" is not trivial either-- often around 200 pounds according to this source https://www.balloon-rides.com/atics-facts.htm#

Re "(it must displace 750 lb of atmosphere, with something not much lighter)" -- if the buoyant lift is to be x pounds (say 750) and density rato of the displacing gas (air) to the surrounding gas (air) is y which you are suggesting is not much less than 1, then doesn't the total weight z of the displacing gas (air) have to be such that (z/y) -z = x?  If y is not much less than one, then the weight of the displacing gas must be much larger than x.  Source cited above lists an envelope volume of 77,000 cubic feet to lift 550 pounds of ballon, plus the occupants which might typcially be 2.

@SDH - I have seen the video now, and the jumper was being an utter assh*le. When the pilot in charge says no, that means no, end of conversation. Unless you have commanded a balloon, you are not really in a position to judge what "stable" means. If you have done balloon jumps, have you ever been the last to jump and have you noticed what was happening to your variometer? The movement might not be rapid (few things in a balloon are) but it is significant and impacts all aspects of flight, including its safety.

@CamilleGoudeseune (ctd) -- this source https://seattleballooning.com/how-hot-air-balloons-work/ suggests a typical max operating temp of 250-300 degrees F.  That would be the temp at the air at the top of the envelope only.  But assuming for simplicity that the entire envelope is full of 250-degree air and we have 77,000 cubic feet of dry air at sea-level pressure, then we still have 0.055907 * 77000 =4300 pounds of air!  (Density calculated by thiscalculator https://www.calctool.org/atmospheric-thermodynamics/air-density .)

(Ctd) So, total weight is way more than double the uninflated weight + occupants!  That helps to explain how it can ever be safe to allow a skydive, or a hang glider drop (google it for some cool videos!)

Please stop guessing and do some research. https://www.youtube.com/watch?v=0ofcl8V9Dws

That's not research, that's a class exercise where the total mass or weight is specified up front.  My point was simply that C.G.'s comment hugely underestimated the total weight of a 2-3 occupant balloon, by incorrectly suggesting that the total weight including weight of air in envelope should be about double the weight of empty envelope+basket+occupants.  Also see links to actual figures for typical envelope volumes within comments.

@PaulSmith I agree the jumper in the linked video was an ass but my question has nothing to do with him. I have jumped multiple times but never the last person to leave. At no point have I ever experienced any sudden motion or instability that the pilot mentions in his explanation. Are you speaking from experience when you say the movement 'is significant'?

(1/2) It is not sudden, nothing in a balloon is. In the 'exercise' I linked to, when 40Kg ballast was released (about half the weight of a jumper),  a 1.5m/s descent (about 300 feet per minute down) was turned into a 0.3m/s ascent (~60feet per minute up) over a period of 5 seconds. That is quite gentle, but definitely significant. Double the ballast loss, and do it when the operator has not planned for it and is not expecting it, and you quickly move into safety of flight issues.

(2/2) If this ass had actually done nearly as many jumps as he claimed, then he should have come across these issues in the past. The fact that he continues arguing with the pilot makes me question his experience. I am not a jumper myself but have heard enough stories of planned jumps being cancelled by the pilot because the conditions did not support a safe release to know that unplanned jumps are just not on.

Video linked above has only limited application to this topic.  Did you notice that the balloon was initially experiencing a constant downward acceleration (not velocity), and likewise ended up rising with constant upward acceleration after the ballast was released?  And there's no consideration given to air drag?  And the 460 kg or 1014 pound balloon is much lighter than any hot-air balloon that can carry more than one person, if we include the weight of the air in the envelope as per my earlier comments.  Sure it gives a slight feel for what happens when we drop some weight but...

Sorry quite flyer, but you are off on a tangent again, what has drag got to do with anything? Watch the video again, an upward acceleration is required to turn a downward velocity into an upward velocity. An 80Kg jumper would result in acceleration that would cause a significant increase in upward velocity. When unplanned, that can introduce a significant flight control and potential safety issue which was the core of the original question.

@PaulSmith -- answered in chat -- https://chat.stackexchange.com/transcript/message/65265222#65265222 , https://chat.stackexchange.com/rooms/151743/discussion-between-quiet-flyer-and-paul-smith

@SDH -- Re "At no point have I ever experienced any sudden motion or instability that the pilot mentions in his explanation" -- keep in mind that even a small vertical acceleration (which will be essentially impossible to "feel"), if sustained, will eventually lead to a high vertical velocity.  As discussed in the chat link, air drag tends to eventually retard the acceleration rate, but still, that basic observation, combined w/ some details given in the other answer, may help highlight the possible issues.  Also some atmospheric profiles tend to promote instability more than others --


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Why should I not use a "lifting tail" on my ultralight design?
In my misspent youth I designed and built many rubber, gas, and RC models. When I wanted good climb and glide I used a aft CG [60 to 80%] and a lifting tail. I had no stability or control problems, though CG was critical. I'm a 75 yr old retired air traffic controller, pilot, flight instructor, A&P mechanic who would like to get his feet off the ground again with a flying wing or tandem wing ultralight.
I don't need speed, but I would like to climb in thermals and ridge lift.  So far a flying wing like the Monarch with 15 to 20hp and a wing loading under 4.5 lbs/ft. and a span of 30/40ft sounds good.  With two lifting surfaces I might keep span under 25ft with a good aspect ratio. Please try to keep answers in laymen's terms.

Why don't you just buy the Monarch composite kit or buy the wood Monarch plans?

How can a youth spent building RC models be a misspent one?

You can use a lifting "tail" for any plane design but it will depend on the design. The only requirement for a naturally stable flying aircraft is that it must have a reasonably good positive static margin. And for that to happen the rear flying surface must have a lower wing loading than the forward flying surface - this does not necessarily mean downforce though that's an easy way to comply with the rule.
In one extreme case you must use a lifting "tail" or the aircraft would not have enough lift to fly. These are canard planes such such as the Rutan Long-EZ. Now, you may make the argument that the thing at the back is not a tail but a wing. Yes, if you were to insist on classifying planes by rigid categories then the rear flying surface of canard planes are normally called "wings" but this mindset will make one blind to what really happens to aircraft stability as you change the planform design.
CG placement, forces on the tail and planform
The correct CG placement for stability depends on several things including the distance between the wings and stabilizers and the airfoil of the wings but the dominant factor is the relationship between the area of the wings and the area of the stabilizer.
Most full-scale aircraft including ultralights tend to have very small stabilizers:

In this configuration the stable range of CG is usually between the leading edge of the wings and the midpoint of the chord. Typically this manifests itself in the rule of thumb of between 25% to 33% of the chord but for actual values you need to calculate the CG based on your desired static margin.
You will notice that most model planes tend to have larger stabilizers. This is especially true for non-scale models and even more so for free-flight models:

In this configuration the CG for the same static margin (stability) appears to be quite far back. In most cases it is 50% of the chord - right near the middle of the wings. It only appears that way compared to full-scale planes or scale models. Aerodynamically the CG placement gives exactly the same level of stability.
This is when some planes can start to be trimmed with a lifting tail. Some real-world planes with large tails such as the Avro Lancaster may in some conditions fly as if the tail is lifting.
As you increase the size of the stabilizer further the CG for stable flight shifts further back:

This is commonly seen in free-flight duration models. Almost all planes with such planform have a lifting "tail". It is rare to see a real plane with this planform however there is one very famous design where you can obviously see the lifting tail - the Pou-de-Ciel:

(Note that the original design of the Pou-de-Ciel has a catastrophic fault where the rear wing interacts with the downwash of the forward wing - later revisions moved the wing back several inches - DO NOT BUILD THIS unless you know all the issues)
If you change the relationship between the forward and rear lifting surfaces further you will end up with a tandem wing. If you've been following the trend at this point I think you can see where this is going: yes, the CG moves even further back:

The typical CG for tandem wings is completely outside the forward wings and sit between the forward and rear wings. Remember, all the planforms so far including this was calculated to have exactly the same static margin - they are all similarly stable.
All tandem planes have both forward and rear flying surfaces configured to be lifting.
Tandems are less rare in manned airplanes than the almost-tandem configuration above. The most well known is probably the beautiful Rutan Quickie:

As we progress with changing the relationship between the forward and rear flying surfaces we will eventually end up with the canard configuration:

Canards obviously have a lifting rear flying surface. So much so that people instinctively refer to them as "wings". And the CG moves further back still (in relation to the forward flying surface, obviously it is in front of the "wings").
TL;DR
As you can see, weather or not your tail is lifting or generates downforce is not simply a choice you make. In some instances you will be forced to make your tail lifting. In some instances it will depend on your current maneuver. It all depends on the design of your plane.
Side note: Why most planes have (relatively) tiny stabilizers?
There are two main reasons why most designers chose to make the tail small:
First is to reduce weight in the tail. The larger the tail the harder it is to get the weight distribution right for the correct CG. Note however that this is mostly due to most designers using conventional wisdom because as we see above if you make the tail large enough you can shift your CG further back for exactly the same stability.
But this leads us to the second reason: to reduce drag. The larger the tail the more drag it generates. Tandems are the worst-case scenario where both the wings and "tail" generate the same amount of induced drag. However several designers such as Burt Rutan have taken advantage of the characteristics of tandem wing planes to produce very efficient designs.

"Why do planes have relatively tiny stabilizers? To reduce the weight of the tail".  This may come to be known as the "tiny down lifting tail fallacy".  As you correctly mentioned, a larger tail will give a larger safe CG range.  Known to the designers of the C-130 Hercules.  A little more drag back there may be an acceptable price to pay for improved safety, especially for pax aircraft.

@RobertDiGiovanni Yes. I think the most significant reason for this is that most people simply follow the rule of thumb of having CG at around 30% chord. It worked in the past so they stick with it. Newer aircraft designs are not shy about using slightly larger stabilizer. Just compare the top view of a 747 and an A380 and you will see how significantly larger the tail of the A380 is in proportion to its wings compared to the older 747. Also, before cheap computing power it was not easy to calculate and optimize the efficiency of an entire aircraft

When I wanted a good climb and glide, I used aft CG, and a lifting tail...

That all goes great, until you stall and have to recover.  Models generally have a much higher thrust to weight ratio than full scale recreational aircraft, which are generally 1:4.
With an aft CG, the tail will tend to "sag" down as velocity decreases, which will result in a higher Angle of Attack, producing a straight velvet-like glide right up to the point of stall.
You definitely do not want this full scale.
Tail heavy models are require less trimming with change of speed, but lack of "static stabilty" is dangerous in slow flight.  Excess power covers a "multitude of sins" with models because adding throttle can increase airspeed almost instantaneously, whereas at full scale
precious seconds can pass having to accelerate more weight with proportionally less thrust.
It is power that enables the climb, not the "lifting tail".  You just do not want one with excessive down force, which can not only be the result of excessively forward CG, but also excessive downthrust angle of a nose mounted prop.
If you want to "get your feet off the ground again", why not go flying with an instructor in a safe and proven design like a Cessna 172?  They've got all the theory figured out all ready, leaving you with a safe and fun aircraft to fly.

With a conventional (downforce) tail, a CG forward of the center of pressure would cause the nose to drop as airspeeds decrease. The less airspeed, the less downforce. This nose-down pitch would increase airspeed until a stasis is reached. The system would be statically stable, decreasing pilot workload.
If you moved the CG further back for a lifting tail, as speed decreased, this would cause the nose to pitch up. The less airspeed, the less upward lift. This nose-up pitch would cause the airspeed to decrease further which would cause more nose-up pitch. The system would be statically unstable, increasing pilot workload.
You could put the CG at roughly the same place as the center of pressure. You could even balance the system to provide equal amounts of lifting force and torque with the CG between the two lifting surfaces. Unfortunately, having two lifting surfaces of different sizes and design would create different proportions of the total force of lift as airspeed varies.
Furthermore, a CG closer to the rudder and elevator (specifically the rudder) would make recovery from a stall and/or spin more difficult. The moment arm of the corrective force would be less.

Lifting tail vs longitudinal stability is already debunked aplenty: https://aviation.stackexchange.com/a/78530/41375

Thank you. Yours are the kind of comments  and ideas I'm looking for. At first glance I'm thinking I need to load up my front wing by moving the CG forward and/or reducing span to ensure it stalls first. It's going to take a few days to digest your ideas.         Thanks agin. Tomtwr

If you want a low sink rate, make sure that induced drag is low. This can best be achieved by a wide wing with a small tail surface. Distributing the wing area between two wings equally will result in one large wing flying in the downwash of another large wing and higher induced drag in total.
With only 25 ft wing span and a total mass of between 120 and 150 kg, your span loading will be 18 kg/m (12 lbs/ft). This is clearly too high for weak thermals, so more wing span is needed for soaring. Aspect ratio is not all - your induced drag is dominated by span loading, so keep that low.
Going with a flying wing as your first design is very ambitious – a decent tail surface makes things a lot easier. Keep the loading of this tail surface low so you have enough margin for maneuvering. Yes, a more forward c.g. location ( = more tail downforce) gives better spin resistance, but by selecting a wing tip airfoil with good post-stall behavior you can gain the same benefit with better performance overall.

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When do the Boeing 737 Next Generation aircraft reach End-of-life?
When can we expect the Boeing 737 Next Generation aircraft to reach End-of-life / stop flying?
The Next Generation is the name given to the −600/-700/-800/-900 series of the Boeing 737 airliner.

......when they’re no longer airworthy and/or the operator discontinues using them?

Given that the 737-200 entered service in April 1968 and there are still a few dozen of those knocking around, I'd say "not for a long, long time".

For an airline, the decision to replace an aircraft is savings-based only. Either because the aircraft cannot be improved in an economical way (compared to buying a new model -- which possibly allows a new marketing strategy), or because maintenance becomes too expensive. There would be also another reason: Passengers don't want to fly on the old model, but this is unlikely to happen.

Some passengers may not want to fly on the new shining model as well.

Airliners aren't given "end-of-life" dates, unlike, say, Windows 95. They also don't eventually become "unsupported", like an iPhone.
A plane will be kept flying while it's safe and economical to do so.
In the case of a particular aircraft, that will depend on many things that have nothing to do with the aircraft itself (oil prices, perhaps) that make it uneconomical to operate.
Older planes are less fuel-efficient, and at a certain point a passenger airliner may be transferred to cargo duties (where fuel is a smaller proportion of the total cost of owning and operating it) and eventually, if no-one wants to keep flying it, it will be retired.
The point at which a plane becomes uneconomical to operate also has to do with how the aircraft is used - an aeroplane that has been through more pressurisation cycles will enter a more expensive regime of checks or maintenance sooner than one that has not (if an aeroplane will need to undergo more extensive checks for fatigue after say 50 thousand cycles, that might be the point at which an operator decides to replace it).
Almost any aircraft can in fact be kept safe to fly (if it were safe to fly in the first place) if you're willing to spend enough money doing so. This is why the comparison with iPhones and operating systems is instructive; probably nobody on earth has enough money to persuade Apple to keep providing iOS updates and manufacturing spare parts for the iPhone 4, but the economics of operating and obtaining spares and doing maintenance on 737s is quite a different matter.
The 737 −600/-700/-800/-900 models are still being produced, and past experience suggests that such aircraft have something like a 30-year working life span.
You never know however if unexpected events (like an oil crisis, or the discovery that airliners cause serious health issues in law-makers' children) will dramatically change the economic environment in which they operate - that's in the hands of fate.

IIRC there is a hard cycle limit that can't be extended with any additional inspections. But it still means each aircraft has its own service life depending on when it was made and how utilized it is.

To give a concrete example of the lack of end-of-life, the DC-3 (mostly built in the early 1940s) is still in commercial service.

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How does Condensation Affect Lift?
Here are two pictures of condensation happening above the wing:

The condensation happens because the pressure drops above the wing which in turn causes the temperature to drop.  When the temperature drops below dew point then the condensation will happen.  The airplane has to be flying slow enough to allow time for the condensation to happen as well.
Here is my question:
How does the condensation affect lift?
I would guess that the temperature does not drop as far because the air would stay at dew point.
Another effect could be that the air is not as dense because the water is now a droplet rather than molecular.
Are there other effects?, and how do they affect lift?

Does flying through clouds affect lift?

“The airplane has to be flying slow enough to allow time for the condensation to happen as well.”  Not true.  Vapes can flash into and out of existence instantaneously under G, and you can see a cone shaped vapor cloud form around a sonic shock wave.

Interesting question, and I will have to speculate when answering.
Condensation changes the lapse rate of air. This makes flying into clouds so effective for gliders. In the early days of gliding some pilots used the higher climb rate in clouds for record flights. But that is not the topic here.
The air above the wing will not cool down as much due to the latent heat freed by condensation. Thus, the air will be less dense compared to drier air which shows no condensation in the same condition. But pressure should not be affected. Therefore, the pressure difference between lower and upper wing surface should not be affected, too.
Lift is the downward acceleration of air, and if this air has less density, lift will also be lower. Now it comes down to how density changes: Moist air is less dense than dry air, so the dry air plus droplets has a higher density. On the other hand, after condensation the air will be warmer, losing some density due to the freeing of latent heat. I would expect that the first effect is larger and lift to be just a tiny bit higher than in a condensation-free condition.


text:
What is the maximum take off climb angle of a Boeing 737 MAX?
This video is making its rounds on social media claiming to show a 90 degree vertical take off:

One can not tell from the video alone what the angle is.

Does anyone have a third party reference for the actual angle for this video?
What is the maximum angle for a Boeing 737 MAX with typical passenger load?
What is the maximum angle for a Boeing 737 MAX especially configured to maximize the angle?

Related: How much thrust is needed by an aircraft to have vertical takeoff? and Could the Boeing 787-9 near vertical takeoff demonstration be performed by Airbus aircraft?.

Typical deck angle with passengers is 15-20 degrees on initial climbout.

The actual angle is 30 degree not 90 ! As it is written in the original video description

Actual angle is 40 degree not 90 !

Even in the original clip you can't see the front of the vertical tail in the climb and that is about 50deg, so you know it is nowhere near 90deg.

That’s a wild demonstration but it’s also not a very realistic  barometer performance for typical 737 operations. The video you see there is a high-performance demonstration flown in an air show or a trade show just for the purposes of entertainment. It involves an airplane that has literally been stripped down of any unnecessary parts, no other personnel or cargo on board except for the minimum required flight crew and barely enough gas to get the performance done and maintain day VFR reserve minimums to get them back to the airport.   The climb there was done in about an angle of 50 to 55° from the horizontal.    Even in these conditions a Boeing 737 does not have a thrust to weight ratio exceeding one and as such the aircraft is in fact slowing down. Still it gives an impressive climb to around 2000 to 3000 feet above the airport for the spectators below.

I don't think it's really been "stripped down", it just has minimum fuel and payload.

It is stripped down. They’re removing virtually everything they can out of that airplane to keep it as light as possible in order to maximize performance.

The plane does have a day job of doing flight testing, the performances are just a side gig. I think it has the full interior installation, but even if it doesn't, it has a bunch of flight test equipment which may end up being heavier.

Pictures of the interior

No it all depends upon whether doing that day. If you’re going to be testing the aircraft they would’ve left the equipment inside the airplane. If they’re doing a demonstration somewhere most likely it would’ve been removed. That is typical of all aircraft use the nearest show demonstrations there literally stripped down of any parts that were that weight and flown in minimum fuel.

Are you suggesting they take all the seats out every time they want to show off?

Sometimes, if the airplane is being used solely for demo purposes, yes.   If it is too troublesome to do that on site or the aircraft is also being used for interior demos they will leave the seats but do things such as remove safety cards and other literature from the seat pockets, no cookware, food, beverages or potable water on board etc.    basically anything that adds excess weight and is easy to remove they’ll get rid of it prior flight.   The jet is not in a condition to be a revenue carrying flight.

There is no need to strip it out for performance. The base model has 60,000lbf thrust pushing 90,000lb plus fuel and payload, so it has a very high thrust to weight ratio to start with. It wont do a vertical climb and still accelerate, but given a bit of room to build up some speed, it it can put its nose a long way up and keep it there. Just because it can be very fuel efficient, doesn't mean it must be!

What I want to now if the rule about parachutes for pitch angles greater than 30 degrees from FAR 91.307c applies ;)

We can find some estimate of the angle from the video, at least at certain instants.
The wingspan of a 737 Max is $W = 35.92m$
The length of a 737 Max 7 is $L = 35.56m$ (assuming this is the correct model).
We can see how that ratio evolves.
When you measure on the video at 0:55 (where the plane seems the most vertical, and with no apparent skewness as both wings are symmetrical), you have $W_{vid} = 7.8cm$ , and $L_{vid} = 4.8cm$ (on my screen), which is a totally different ratio because the plane is inclined.
So from the camera's perspective, the inclination angle at that moment is $asin(L_{vid}/W_{vid}) = 38deg $
To have the inclination with respect to the ground, you would have to add the camera's own angle with respect to the ground, which we don't know at that instant (could be 10-15 degrees, which would give an inclination within the range given by @carlo-felicione).

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How do I distinguish 2 fixes with the same name?
I see two fixes in the CIFP file that have the same name PERSN.

Given 2 fixes that have the same name but are located in different regions (defined by ICAO code) how does airplane/pilot distinguish between those 2 when respecting flight plan?
According to FAA Procedures for Handling Airspace Matters

e. AIM [Aeronautical Information Management]  must  not  duplicate  any  radio  fix, waypoint,  marker  beacons or  compass  locators names.

how come that those fixes have the same name?

That is a related question, not a duplicate. By the way, the 2 fixes in the example do not share the same name, even though they are very similar.

If you try to flight plan with https://skyvector.com/ from say KBOS to KLAX (Boston to LA), K70 and PERSON are not valid waypoints.  K40 and PERSN are,  with PERSN sort of enroute, and K40 way up in Alaska. So a pilot would know something was off.

@bogl 5-character name which is marked in ARINC docs as Waypoint Ident is PERSN for both. I am trying to understand what makes a fix unique as I though it was id number which is unique.

The answer is embedded within the question:  The pilot is able to distinguish between the two fixes because they are in different regions.

Is it possible for IFR system to put fix together with ICAO Code for region? Usually for commercial flights I see plain IDs without any region code (e.g. in FPs from flightaware24)

That sounds like a good question for a controller or someone who enters flight plans into the ATC system.  I'm just speaking from a pilot's perspective, and it should be pretty simple to recognize that one of your waypoints is way off.


text:
What kind of flight counts toward the commercial 100 nautical mile cross-country flight requirement?
A question regarding the dual 100nm day/night 2 hours.
Does a 130nm flight with 1 stop halfway (over 2 hrs) count?
1 leg of 80nm, 2 leg 50nm.
Total time 2.5hrs.

Hi EchoS, welcome to Aviation!  For questions like this please provide a region that you're interested in.  I assume you're asking about the USA, but it's much easier to provide an answer if you say so specifically.

If you're asking whether cross-country flights in the US must be one continuous leg or not, this question might be useful.

What kind of Flintstones cardboard box are you flying that takes 2.5 hrs to cover 130 nm?

Your question didn't give enough information for a 100% correct answer, so I'm going to make a few assumptions:

You are asking about FAA certification in the USA
You are asking about part 61 training, not part 141, 142, or anything else
You're asking about a single-engine commercial rating (this one is less important than the other two)

With those assumptions in mind, the controlling regulation is 61.169(A)(3)(iii-iv), which says (emphasis added):
(iii) One 2-hour cross country flight in a single engine airplane in daytime conditions that consists of a total straight-line distance of more than 100 nautical miles from the original point of departure;
(iv) One 2-hour cross country flight in a single engine airplane in nighttime conditions that consists of a total straight-line distance of more than 100 nautical miles from the original point of departure;
So you need to ask yourself:

Did you fly at least 2 hours?
Did you land at an airport at least 100 miles away from the original point of departure?
Did you do the entire flight during the day/night?

If so then you're good.  If not then no.

Thanks Steve, I'll be more specific this time. You're right in all 3 assumptions. FAA Certification, Part 61, Commercial Single Engine Land. The example: I depart at night with an instructor, for a 90nm flight (straight line), then fly another 50nm leg. I end up landing in an airport which is 140nm from the origin. None of the legs were 100nm, but I landed in an airport 140nm away from the origin. Total flight duration 2.5hrs. Am I good or not?

No, because you didn't go at least 100nm before landing.

@EchoS Yes. As Steve V said, you literally met the requirement of the FARs so you are fine. By the way, make sure the instructor writes in your logbook that the flight is to satisfy  61.169(A)(3)(iv). For my commercial night I did just about the same thing, except that it was three legs and 131 nm. We made dogleg stops to get night current and also to make the total flight more than 2 hours.

"from the original point of departure" implies there can be intermediate points of departure, i.e. multiple legs. If they meant one leg of 100nm, they would have worded it differently.

Be careful here, you're going to have to full the requirements to the exact letter of the law or the FAA or other regulatory bodies will not approve your application to take the practical exam.
61.129(a)(3)(iii) and (iv) or 61.129(b)(3)(iii) and (iv), dependent on the aircraft category or class you are applying for, state:

One 2 hour cross country flight in a single/multi engine airplane in
  daytime conditions that consists of a total straight line distance of
  more than 100 nautical miles from the original point of departure.

and

One 2 hour cross country flight in a single/multi engine in nighttime
  conditions that consists of a total straight line distance of more
  than 100 nautical miles from the original point of departure.

Get out a chart plotter and a VFR sectional and draw a straight line from your point of departure to the destination airport.  If this straight line exceeds 100 nautical miles and is made during the conditions you are required to satisfy for the rating, then the conditions are met.  At least one leg of the proposed cross country flight must have a straight line departure to destination distance of at least 100 nm, the rest of the cc is up to your discretion.

Carlo You almost got it right. "Total straight line distance" is not the same thing as "one leg".


text:
Why do helicopter operations avoid IFR?
In almost any airplane commercial operation, unless you’re doing something very short haul daytime ops, an Instrument Rating is all but required with airplanes (it’s even in the FARs). Generally the weather won’t stay nice enough for you not to use it, and most airlines and other commercial operations will avoid VFR all together, leaving VFR mostly for recreational pilots.
In helicopters however it seems much different. Even in professional settings like news services, 135s Helicopter operations, and even essential services like fire rescue, will try and stay VFR at all costs, and will avoid flying if the weather is not VFR, even if they have an Instrument Rated helicopter pilot. VFR has a lot of limitations that would have a large chance of losing money for a helicopter service.
Why is IFR so much more taboo and/or dangerous in helicopters compared to airplanes?

Your premise isn't true. There are many happy pilots without instrument ratings and they are able to do plenty of flying.

As a controller I talk to helicopters flying IFR on a daily basis

The news service wouldn't send a helicopter out to film the inside of clouds.

Although there might be a rare few commercial pilots out there that are not instrument rated, I personally don’t know any helicopter pilots exercising the privileges of their commercial license that are not. Not being Instrument rated would be very limiting when carrying persons or property for hire. As far as nice weather, my location gets almost 300 days of sunshine a year. Wind and turbulence have far more affect on flying than IMC does here. The same weather that would ground a Bell 206 would also ground a Cessna 182.

Helicopters are used in different scenarios than fixed wing aircraft. Flights between (for example) the ILS-equipped airports of Berlin and Cologne are typically performed by fixed wing aircraft. Helicopter are typically used where no runways are available. For example, D-HHSD (yep, the very helicopter we saw in the first TV series "Airwolf") was used for a flight from Berlin to Cologne for a transport directly to the University Clinic Cologne. An IFR-only landing might not feasible there (https://www.medfacilities.de/projekt/herzzentrum/). It crashed on the IFR return flight.

Helicopters do not avoid IFR. They will, however, try to avoid IMC for safety reasons. The same is true for airplanes in the same weight class. Just because the pilot is IFR rated and the aircraft is IFR certified, it does not mean they will always fly IFR, whether they are fixed or rotary wing. Light aircraft in general don’t do well in extreme IMC, whether they are fixed or rotary wing. And, many of the same FARs in Parts 61, 91, 119, 135, 136 and 137 that would require an instrument rating apply almost the same to rotary-wing as they do to fixed-wing.
The main difference between fixed and rotary wing would be their environments of operation. Altitude would be a major factor. The majority of airspace below 1200-1500 feet AGL is uncontrolled airspace. You can not get an IFR clearance unless you are in controlled airspace. Since Part 91.119(d) allows helicopters to operate at reduced altitudes and distances from persons or property, they will specifically be used in applications not conducive to IFR flight.
If a helicopter were to fly IFR,they would be restricted to at least an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown by Part 91.177. This would greatly reduce the usability of helicopters. The increased restrictions of IFR flight means that there is much more opportunity to make money in VFR flight with a reduction of risk.

§91.119 Minimum safe altitudes: General.
(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA; and
(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.

As far as your assertion that commercial fixed wing operations are almost exclusively IFR, that is not a completely true statement. While I will agree with you that may apply to Part 121 operations. That is not necessarily the case for all other operations. Many commercial operations depend exclusively on flying VFR.

Which airspace class forbids IFR flight?

@VladimirF - You can not receive IFR clearance in uncontrolled airspace. Class G airspace is uncontrolled airspace. While flight in IMC in Class G airspace is not expressly forbidden, ATC does not provide aircraft separation. See the following link regarding the FAA’s opinion on legality.  http://www.ntsb.gov/legal/alj/OnODocuments/Aviation/3935.PDF

It did not occur to me you were thinking of USA because I remembered US was class G below FL180. Wrongly. However, it is still possible to fly IFR from uncontrolled aerodromes even in USA ifr-magazine.com/charts-plates/uncontrolled-ifr "If you have received an IFR clearance at a Class G airport and wondered at the “upon entering controlled airspace…” phrasing, this is the reason". There other less populated areas in the world where there is much more uncontrolled airspace. There also used to be class F.

@VladimirF - Yes, I was thinking of the US. You can file your IFR flight plan from a Class G airport and/or to a Class G airport. You can get your clearance and release by phone, RCO, etc. But, you will remain uncontrolled (and VFR supposedly) until reaching controlled airspace. ATC separation services begin upon entering and end upon leaving controlled airspace. Once your destination is insight, ATC will issue you a clearance for a frequency change to CTAF, remain VFR, and the choice to close your flight plan either in the air prior to changing frequencies or on the ground by phone, RCO, etc.

@VladimirF - Hence the “upon entering controlled airspace…” phrasing in the clearance. ATC has to make it clear to pilots that their IFR clearance starts in controlled airspace. Pilots are on their own until then. And, since the “See and Avoid” doctrine requires the pilot to be able to see, flying IFR in uncontrolled airspace can be strongly discouraged using Title 14 CFR Part 91.13 as being careless and/or reckless.

There is sort of a gray area where IFR pilots are protected from other IFR pilots very close to their arrival/destination airports, because if a pilot gets their IFR clearance on the ground, or elects to retain their IFR clearance to the ground, the ATC will "block" the surrounding airspace for other IFR flights. But this only applies to IFR flights that ATC is actually controlling; a pilot could fly under IFR but without a clearance in Class G airspace without contacting ATC, and the block would not provide separation from such a pilot.

@VladimirF I may be doing it incorrectly, but when giving a clearance off an uncontrolled airport I don't say "upon entering controlled airspace" unless giving an explicit turn. Otherwise I simply issue the clearance "as filed" and allow the pilot to maneuver as needed, but I still consider them operating under an IFR clearance... I just know that 1) they're going to climb into controlled airspace relatively quickly and 2) I won't give them a vector until they reach the MVA. Note that this only applies to a departure off an airport; I won't issue a clearance to an airborne pilot below the MVA.

@randomhead - Thanks for the ATC perspective. Correct me if I am wrong. But, a pilot is not required to have an IFR clearance to fly in Class G. And, although they may be flying on an IFR flight plan, their clearance can not start until the aircraft reaches controlled airspace. ATC may know their departure and arrival airports per the flight plan. But, can ATC provide the necessary aircraft separation for actual IFR clearance in Class G? Although the issue still remains that IMC flight in uncontrolled airspace, though legal, is careless and reckless (Part 91.13).

As I said it's a gray area. The .65 4–2–8d only says we have to withhold an IFR clearance if the pilot is unable to maintain their own terrain sep until the MVA. Otherwise we can still give the clearance, but again the assigned altitude will be at or above the MVA which will be in Class E by necessity. See also 4–3–2b1 and 4–3–2c1(c); take those together and you see we can issue a clearance that is effective within Class G. You're correct that traffic separation is not guaranteed, because G is uncontrolled.

Helicopter IFR operations do exist, but the short answer to the thrust of your question is: helicopter IFR is inherently more dangerous than fixed-wing IFR due to the lack of stability. A properly trimmed fixed-wing aircraft in good conditions could fly upwards of 30 seconds without pilot intervention, whereas a helicopter needs almost continual control inputs, thus leaving less margin for error.
To help overcome this risk, the certification requirements for helicopters are more onerous than fixed-wing aircraft, such as stability augmentation systems and autopilots. The effect of this historically has meant only the bigger and more expensive helis were actually IFR capable, so the pool of rotary pilots with IFR experience is much lower than fixed-wing pilots.
Finally, the obvious risks associated with helicopter operations - low to the ground, close to buildings, etc - do not work well with bad weather.
Here is a really interesting article which talks about these problems in depth.

Good article. Although the Robinson R44 is IFR equipped for training IFR, it is only certified to fly in VMC. You can get the rating and currency under the hood, but no actual IMC time.

Oh my goodness, I do not understand the lack of knowledge of the logic for use of helicopters in IFR operations among my fellow pilots. As “Fletch” said in the movie of the same name, “guys, do I have to teach you a lesson!?”  Haha.
Okay gents, here’s the condensed version. And let’s use Part 135 helicopter air ambulance (HAA) operations as an example of effectively utilizing helicopters in an IFR capacity.
Bottom line, up front—-an IFR helicopter can do more flights than a VFR-only rotorcraft. Lots more. And I’ll argue, far more safely.
Here goes the basics… first off, both a VFR helo (helicopter) and an IFR helo can conduct VFR revenue flights. So far, so good on my logic, right? But next up are the additional flights you’ll capture with an IFR helo: (1) the actual IFR flights you plan and fly and (2) the “gray area” flights that may not require an IFR helo, but are often declined by prudent pilots when weather is too close to VFR minimums to safely prosecute the flight. Likewise, oftentimes, the gray area flights add up to a few flights per pilot per month (x4 pilots at a given HAA base) which results in somewhere between 4-6 flights per pilot that can be flown in VFR conditions (if desired) but when the pilot can easily pickup an IFR clearance if desired or when prudent (maybe conditions worsened or perhaps one piece of the VFR puzzle looks questionable). In any case, the additional IFR flights and “gray area” flights add up to a “bunch” of additional flights per month, a “load” of additional revenue per month, and of course altruistically speaking, a greater patient care opportunity for the residents of each IFR helicopter’s radius of action.
Speaking off the cuff, I’d wager that each SPIFR (single pilot IFR) at every base that actually has weather conditions frequently experiencing IFR weather conditions may pickup anywhere between 10-15 additional flights per month.
Of course, not every location has IFR weather very often, but that’s a scenario that is outside the scope of my argument. The thing is, SPIFR opening HAA helos results in far more flights per base in areas with actual weather conditions
Is it easy? Yes. Terribly easy. And is it fun and safe flying? You betcha. This is because SPIFR pilots are flying very well equipped IFR machines (e.g., EC135s, EC145s, etc.) and the airframes are just absolutely solid and brilliantly designed for every aspect of IFR flight.
Stability and automation in SPIFR is held at a high standard and frankly goes beyond the hurdle of safe flying to the higher level of enjoyable safe flight operations. It’s just as easy as I’m saying. Otherwise, it just wouldn’t be right or worth the hassle, expense or trouble.
How is it done? The SPIFR pilot is well trained in the helo, yes. But the methodology for SPIFR operations is kind of a key to each pilot’s success. Every SPIFR pilot becomes aquatinted with the IFR route they may need on any given instance and become intimate with every IFR departure type at every likely departure point (certainly his own base or airport), then studies every IAP (instrument approach procedure) he or she may need to execute, then starts the shift with “what can I do today? Where can I go? How can I potentially accommodate each county, city, EMS agency, fire department or hospital—that might need IFR helo services?”
Yes, it becomes that deep of a thought process. The SPIFR runs scenarios and looks at potential leg patterns (e.g., IFR departures,IAPs, transitions, fuel spots and IFR alternates). And before you think, “there’s a lot of flying you can’t do, mister”, I’ll go ahead and agree but add, there’s a lot of revenue flying that can be flown safely that would otherwise resulted in a non-flying alternative.
The point of my argument is not ‘whether or not every flight request “should fly” with respect to medical conditions’, but rather, ‘what is the mindset of HAA SPIFR operators and how it’s thought about by those who do it. It’s really just as easy as I’m telling you.
Sidebar: I’m not going to delve into the obvious increased safety factor of having IFR systems and autopilots as well as a 2nd engine to fly you away from a forced landing or continue flight with the remaining engine after one engine fails. Nope. We’ll save that argument for a different day.
But if the question is, “would an IFR helo bring my HAA company more patient care and more revenue whilst increasing the safety of my crews simultaneously, the answer is an easy— yes.
Tim Grimes
ATP helicopter
12 years SPIFR

The third paragraph seems to summarize the content of this  answer.  Very interesting -- but aimed perhaps at a different question than the one that was asked?

There's another consideration: cost. Specifically, company overhead cost.
If you're flying commercially (let's focus on Part 135 to keep it simple), you cannot just "do an IFR flight" if/when A) the pilot is qualified and current, and B) the helicopter is certified and capable.  Just because the FARs say you can, doesn't mean you can.
Every operator (no matter what Part) must have an Operations Specification (OpSpec), issued by the FAA. These OpSpecs will go into the minutia on what you can or cannot do, what airports you can or cannot use, if/when/how you can or cannot conduct an IFR flight, etc.  (hundreds of pages of this stuff). These OpSpecs are then reviewed periodically, revised as needed, and "continuously monitored" by that operator's POI (Principal Operations Inspector) at the FAA.
So, back to the question (kind of). In order to conduct an IFR flight, it must be specifically stated in the OpSpecs that the company can do that IFR flight.  In order for that authorization to make its way into the OpSpecs, the FAA must see that the company is maintaining a certain level of training for the pilots and a certain level of maintenance for the helicopters. These are expensive!! The costs of these mandatory requirements are not worth the benefit of being able to do the random IFR flight on those blue moon occasions when your point-to-point flight is going to be going through IMC and you need to get "into the system".
It's simply much more cost effective for most helicopter operators to say they just won't do IFR flights, and have that authorization withheld from their OpSpecs.

authors: Vladimir F Героям слава


text:
How do I estimate piston aircraft fuel efficiency?
Is there any way to figure out or calculate fuel consumption of piston engines for aircraft? I'm looking at different engines in a book, and while jet engines have useful values like thrust, bypass and pressure ratio, piston engines only have power output and weight.
Propeller efficiency plays a role here, I guess, but it's not at hand so I assume all engines use the same propeller in this case.
Would the easiest way to calculate such a number be to find the maximum fuel capacity and range and from there find out how much fuel each engine uses per hour?
I've asked a similar question before about turbojets and turbofans and received good answers - sadly, piston engines are completely different...
EDIT: To clarify, I am looking at airliners powered by piston engines and wish to compare them.

Do you mean in general for a model engine, or for a specific aircraft? Piston aircraft have fuel consumption charts in the POH that tell you how much fuel in gal/hr you will use at different power settings and different conditions.

To clarify, I am looking at airliners powered by piston engines and wish to compare them. Edited into original question.

Yes there is a simple rule of thumb for what is known as Specific Fuel Consumption that you can apply to estimate the fuel burn of most piston engines:
An air cooled carbureted engine, leaned, will burn roughly 0.44 to 0.45 lb/hp/hr.
With Fuel Injection, a bit less, maybe 0.41 or 0.42-ish.
Cars with electronic sequential fuel injection are in the high 0.3s.  Diesels are in the low 0.3s
2 strokes are up in the mid 0.5s.
Turbo props are very thirsty and are in the .6 range.
Take a Cessna 150.
75% cruise is 75 hp.
X 0.45 = 33-34 lb/hr.
= 5.6 usg/hr.
Which is about what you will find in the POH, or close enough anyway.
Want to know the fuel burn of an R-985 powered airplane?
450 X .75 = 337 hp
@ .44 lb/hp/hr = 148 lb/hr
= 26 usg/hr, which sounds about right for something like a DeHavilland Beaver.

Thanks for your reply. This means that by estimating the SFC and cruise speed %, I can get the (estimated) actual fuel usage per hour? Other than engine type, what might affect SFC?

If it's for a big radial say, whether the engine is in auto rich or auto lean.  In auto rich SFC would be maybe .5-.6 because you are using fuel for cooling. Also, turbo charging helps reduce SFC since you are recovering lost energy that would be wasted.Also Power Recovery Turbine systems as used on the R3350s and other big radials, which recovered about 300 hp from the exhaust, reduced the R-3350's SFC into the high .3s. Was one of the most fuel efficient aircraft piston engines ever made. 75% power cruise of a Super Constellation, I'd use an SFC of around .38. Using 3500 TO hp, 665 usgal/hr.

With 80 ppl aboard going 340mph, the S Connie would be burning 8.3 gal per pax, getting about 41 per-passenger miles per us gallon.  I don't think any jet can come close.

@MatiasLq: Speed will not affect fuel consumption per hour. But altitude will, and now you need to know how much the engine is super- or turbocharged. Generally, as density decreases with altitude, fuel flow will do so, too.

You don't really need to calculate it, manufacturers take care of that for you (for every plane built in the past half century or more). These numbers are available in the POH for the given aircraft but you will need to chose your aircraft as engines may have different characteristics based on installation.
For example here is the POH for the Warrior I fly you can find the fuel burn info and performance data starting on page 5-12.

Another example, for an O-360 (180HP) in a Cessna Cardinal. I lean to about 10 gallons/hour once leveled off in cruise. Maybe somewhat richer in combination with opening the cowl flaps in hotter weather to keep cylinder head temperatures to 380F or lower.

Thanks for the reply. I was looking more towards airliners rather than sports planes. Not many commercial piston aircraft have been manufactured the last 50 years, so it would seem difficult to look for/purchase POHs for many different aircraft.

@MatiasLq old POH's for planes of that size are not terribly hard to come by and have plenty of engine data the layout and organization is more or less the same as a small aircraft. Most GA planes that are commonly flow (172/Cherokee) were designed ~50 years ago anyway.


text:
How does change in airspeed affect pitching moment?
In aircraft with conventional elevator control, if I increase the speed, the horizontal tail sees higher velocity and hence higher lift. Does it mean that increasing airspeed creates nose down pitching moment (i.e. decrease in AoA) and vice versa ?
If So, it means if i want to increase the speed keeping altitude constant, the aircraft automatically gets trimmed at new trim AoA or i manually have to change the AoA using elevator?

Note that, depending on location of the cog the horizontal stab is providing lift in a downward directing, thus an increase in speed would create a pitch up rather than down

How exactly do you plan to increase speed? You can add thrust. The aircraft is trimmed for a specific airspeed (normal flight assumed) so the added thrust will introduce a climb.
If you want to increase speed in level flight, you trim down (the engine power force vector rotating down, shrinking the vertical component and increasing the horizontal, so converting more engine power to forward movement and losing a little lift) AND add power to compensate for the lift loss.
It follows that there is an optimum: when there is no more vertical component, you cannot increase forward component of the engine force vector. But by that time the aircraft is already converted into a rock (no vertical force = only gravity = fall).

If you increase speed, you do not only increase the aerodynamic forces on the tail (be they lift or downforce, doesn’t matter), but on the wing as well. The weight of the aircraft will stay the same, however (if we neglect the fuel consumed during acceleration).
This increase in wing lift needs to be compensated by a lower pitch attitude of the aircraft, so the tail surface will see a reduction in angle of attack which more than compensates for the increased dynamic pressure. As a consequence, a naturally stable aircraft will experience a nose-up pitch moment change when being accelerated.
In order to trim the new speed, the elevator deflection needs to move to a more positive setting (= trailing edge down).

Do u mean that to trim the a/c at any new airspeed, only changing the pitch angle by elevator is required and no throttle change is done. Throttle change is done only to change flight path angle?

@Swati. well. if you change trim, the aircraft will assume the new speed, but at that speed drag is different. If you don't adjust throttle, your potential energy will change by the difference resulting in climb or descent (there is a particular speed where drag is minimal, so we can't say slowing down always results in climb).

A simple answer:
Normally the wing has an upward force (lift) and the tail has a (usually) downward force (negative lift if you will).  So as the aircraft increases airspeed, the lift of the wing, and the (usually) downward lift of the tail will increase.  There are other factors which include the engine / prop orientation, center of mass, and so on.
The issue which makes a clear, always true, answer elusive is that the lift profile of the wing and of the tail are not the same.  And of course there are complications from trim tabs and the like.
So when a designer considers pitch changes with airspeed changes, they usually try to minimize the effect to provide longitudinal  stability. (Exceptions can be fighter, aerobatic and fly-by-wire configurations.) Another normal design goal is to have a lower angle of attack on the tail, than the wing. They can only be identical at more than one point, unless there is an unconventional or active flight control.


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What does stick fixed and stick free longitudinal stability mean in simple terms?
What is the significance of stick fixed and stick free terms in aircrafts with mechanical control and modern fly-by-wire control?

Trim the aircraft. Do a "doublet" (means stick forward and back to the trimmed position for a short moment, or rudder deflected and neutral again) to upset the trimmed state.
Now watch the response while you keep all control surfaces at their trim position, making sure the control surfaces don't move when the aircraft goes through its motions. This is quite easy on GA aircraft and big aircraft with powered controls (if you do this on an Airbus, switch it to direct law).
Next, repeat the maneuver but let go of the stick respectively the pedals. Watch how the stick or pedal position moves with angle of attack respectively sideslip changes: This is your control surface floating freely. Again, watch the response. Normally, it will now take longer for the resulting oscillation to die down and the oscillation frequency should be slightly lower.
The first is the fixed-stick aircraft stability, the second the stick-free stability. Normally, stick-free stability is lower and proportional to the stick forces when flying away from the trim point. The difference between both is determined by the hinge moment coefficients of the control surface. For the elevator the floating angle $\eta_f$ is
$$\eta_f = -\frac{c_{r0}+c_{r\alpha}\cdot\alpha}{c_{r\eta}}$$
with $c_{r\alpha}$ the hinge moment coefficient over angle of attack $\alpha$ and $c_{r\eta}$ the hinge moment coefficient over control deflection. Both coefficients are normally negative and the absolute value of $c_{r\eta}$ is larger that that of $c_{r\alpha}$. When $c_{r\alpha}$ is zero, both stabilities are equal because the floating angle is zero.
Another way to study stick-fixed pitch stability is to trim the aircraft at two different speeds or load factors and to measure the trim change necessary between trimmed state 1 and trimmed state 2. The result is the speed stability rsp. the maneuver stability.
Stick-free stability is only relevant in case of manual controls. FBW aircraft only exhibit stick-fixed stability except for the very unusual case that the control surface actuators are overpowered by the aerodynamic forces.

"hold on to the stick rsp. the rudder pedals" - What do you mean by "rsp."? I'd imagine there's something obvious to your German/English speaking brain that's not making the translation to my English-only brain.

@FreeMan respectively - please suggest a better alternative! I am happy to edit the answer.

"hold on to the stick and the rudder pedals" ... "repeat the maneuver but let got of the stick and the rudder pedals"? As I'm reading it, I think that's what you mean. Maybe "stick or rudder pedals", I'm not sure since I only know the grammar, not the flying/physics...

@FreeMan: No, it is not both but either - or. The technique works best when there is only one disturbance, either elevator or rudder.

Consider an aircraft in stationary, horizontal, trimmed flight. Longitudinal stability could be described as the tendency for the aircraft to return to its original position after an external event has changed the Angle of Attack $α$.

Static stability: with the pitch stick held at a fixed position, an aircraft is statically stable if the aircraft has a tendency to return to its original position after an attitude change by an external event. Without having to give a stick input. So a wind gust causes a nose up moment, changes the aircraft attitude, and the increased attitude causes an opposing aerodynamic moment that drives the aircraft back to its original position once the gust is gone. This can only be achieved if the centre of lift is aft of the centre of gravity:

$CN_2$ > $CN_1$ when $α_2$ > $α_1$
Dynamic stability. So our statically stable aircraft is driven back by aerodynamics to where it was, but what happens then? If it reaches neutral, then overshoots further to the other side, then back through neutral and further yet  again, the response diverges and the aircraft is dynamically unstable. Not a good situation because in order to return to trimmed flight, we need to generate periodical stick inputs that counteract the natural response. We would like the aerodynamic forces to take care of this.
But what happens if we're dynamically stable with fixed pitch stick but don't hold it in position during a periodic movement? Does the returning angular motion cause an elevator deflection that counteracts the motion, or one that amplifies it? That depends on the behaviour of the elevator after an external event has caused an increase in $α$. Does the stick start to wobble, just like the aircraft did, and what sort of wobble is it? One that returns to where it was or is it diverging? This is a function of the sign of the hinge moment of the elevator. If we have an elevator that floats up after an increased $α$, it will tend to increase α$$ even further. So that arrangement will reduce the static stability of the airframe.

The above is a nutshell description of aerodynamic stability. There is more to consider, like stick manoeuvre deflection and force stability, but that may make this post too lengthy. However, we can see that for an aerodynamically stable airframe, we need an aerodynamic moment that counteracts our motion. OK for a Cessna but not desirable for high manoeuvrability aircraft like fighters. For these, we want the aaeroforces to help us make the turns - but that leaves the pilot with an unstable equilibrium, like trying to balance on top of a big inflatable ball. Tiny stick inputs are constantly required to fly in a straight line, because aerodynamics constantly want to deviate us from the trimmed position.
And that is where fly-by-wire comes in. It uses a computer to generate the small inputs required to keep the airplane neutral - constant small deflections that are not noticeable to the pilot, he only experiences a stable aircraft. Give a stick input, and all aerodynamic forces cooperate to make a rapid manoeuvre.
I've heard that Formula 1 driver Michael Schumacher had his car tuned just like that: directionally unstable so able to turn quickly but requiring constant small corrections at the steering wheel. Not everybody's cup of tea.


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What is advantages of a wing mounted engine in comparison to tail and fuselage mounted engine?
What is advantages and disadvantages of wing mounted, Tail mounted and fuselage mounted engines?

https://aviation.stackexchange.com/questions/1397/how-does-the-mounting-location-of-a-jet-engine-affect-aircraft-performance/


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How does an API (air position indicator) work?
I was looking for WW2 aircraft indicators and I found one navigation equipment called API (air position indicator).
But I couldn't find how it works exactly????

Can you be more specific which kind of information are you looking for? This description: http://timeandnavigation.si.edu/multimedia-asset/air-position-indicator (first search result, "It took inputs from airspeed sensors and gyro magnetic compasses and continuously computed latitude and longitude. ") seems to cover the basic principle quite well. What else do you want to know in particular?

How does it get the latitude and longitude from airspeed and heading??? by which formula?

It has the departure point's Longitude and latitude entered before takeoff. The uses airspeed and heading to calculate location as it flies along.  A crude but effective approximation of actual position. The pilot uses map references for terminal guidance.  In effect, a a much more inaccurate version of a modern INS.

This question would be even more interesting if it mentioned some examples of aircraft that actually used this type of instrumentation.  So basically this system automates the ded. reckoning process; very interesting.

Insert joke here about the "Air Position Indicator" being another name for the blue half of the Attitude Indicator.  And the "Ground Position Indicator" being another name for....

The API uses not only heading and airspeed but they are fed to mechanisms within the instrument that solve the navigation problem using sine, cosine and secant functions. The output was latitude and longitude from the departure or a known fix (geographical position) to give the no wind position of the airplane. This was then used by the navigator to give a dedreconing position by plotting the last known wind vector from the no wind position.

These links (to download documents) give interesting information about a "Ground Position Indicator" from 1953, which sounds like an "Air Position Indicator" with added capability to correct for assumed wind speed and direction. The device took into account the spherical shape of the earth, but apparently did not incorporate acceleration sensors as per a true Inertial Navigation System.
(Click on all 3 links under "Ford Instrument Company")
http://analogmuseum.org/english/library.html
And here is a link for information about the "Air Position Indicator":
https://timeandnavigation.si.edu/multimedia-asset/air-position-indicator
Apparently the only inputs to the "Air Position Indicator" were true airspeed, gyro-magnetic compass heading, and initial lat/lon.  It automated the ded. reckoning process, taking into account the spherical shape of the earth, and magnetic variation, but assuming no wind.  Apparently the pilot or navigator then corrected the result for assumed wind speed and direction.


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How IATA Code is requested or assigned to an newly built airport ?
We have an upcoming airport in our state. An ICAO code is already in place but who and when will the IATA code will get created ?

You request one from IATA.

@RonBeyer: Interestingly you are supposed to be an airline business to request a IATA airport code (form).

@mins Absolutely, until the airport has a commercial servicer, it shouldn't have an IATA code (IATA stands for International Air Transport Association). It really doesn't make sense to have one for any other purpose.

Airports are assigned IATA codes when an air servicer (airline, cargo operator, etc) requests a code for an airport. The code is requested by filling out a form on the IATA web site.
This is not done by the airport management because it is meant to be for use by a commercial operation, and not the airport itself. IATA codes don't really have a lot of meaning to pilots and other general airport users. They are of use to air travelers and cargo operations as a means to identify airlines, destinations, baggage codes, accounting codes, and traffic documents. It is designed as a standard for industry wide usage, otherwise airlines may use many different codes/abbreviations for the same thing (an airport for example) that don't mix well with other airline systems.
So after your airport is built (or while it is being built), an airline or cargo operation may (or may not) request an IATA code for the airport. Participation in the IATA program is purely voluntary, and it may be that your airport doesn't handle services that conform to IATA.


text:
Hot air balloon toilet?
I have never been in Hot air balloon
But today I was wondering, what if someone in Hot air balloon wanted to pee?
how will they do it??
if there are no toilets in there, can they just do it from there to the ground? what if there was people down there?

Overboard or a plastic bottle.

We normally look for more detail than this but, honestly, I'm not sure there's much more to say, here.

The same way people solve this problem on a small boat - overboard.
(Fun fact: most large boats that do have toilets, flush them overboard as well. Our civilization isn't that averse to treating one another to our waste.)
If there are people down there, they might get a few drops of golden shower. But considering the small number of balloons in the air, your odds to get hit by bird droppings are a lot higher.

In civilized parts of the world, boats are not allowed to flush the toilet any more, and holding tanks are mandatory.  ;)

Then what I've seen of Europe (the Med) isn't civilized ;). The holding tanks get flushed out to sea the moment the boat is leaving the harbor. At least by most owners.

@Therac there is typically a minimum distance off shore at which you can pump overboard. In the US its only 3 miles which is not terribly far. In Europe the regs range from 2 miles to never but it also seems that not all boats are required to have holding tanks which makes it tricky to enforce in some places.


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Why don't I see this kind of split ailerons on aircraft?
Why don't I see split ailerons that work like spoilers going up and like normal ailerons going down? Isn't that a good way to get rid of adverse yaw?

Adverse yaw is caused by difference in drag for aileron up and down positions, aileron down has more drag than aileron up; in this configuration when aileron is up additional drag is created because of split. –

Could you add to your question why you think this would reduce adverse yaw?

Adverse yaw is caused by difference in drag for aileron up and down positions, aileron down has more drag than aileron up, in this configuration when aileron is up additional drag is created because of split.

But for the same roll authority and less total drag, why not just use some differential, i.e. increase the movement of the upgoing aileron and decrease the movement of the downgoing aileron?

I believe that the complicated actuating mechanism that such a setup would require simply would not make up for the minor gains in countering the adverse yaw.

When Sanchises asked you to 'add to your question why', they were not looking for a comment, but literally for you to add why you thought it would reduce adverse yaw Into The Body Of The Question. I went ahead and did that for you via an edit (to "teach by doing.") Hopefully you'll get an answer that you find useful.

It is possible to produce a split aileron, which would indeed be(much) less susceptible to adverse yaw. But there are downsides as well:

It would create complexity, and therefore higher risk of failure. Which is not wanted in the flight control surfaces, the input to controlled flight safety.
It is also much less efficient at producing roll: a significant portion of the rolling moment has disappeared, due to the lower surface not contributing to the up flow diversion.

Image source
Above pic shows how adverse yaw is eliminated without adding complexity: the frisé aileron has a lower corner that pokes out when deflected upwards, thereby increasing drag.
As stated in @Mark’s answer, the later versions B-52 had replaced its small ailerons with spoilerons. Not for reducing adverse yaw, but for eliminating aileron reversion. From the wiki:

Because of the B-52's mission parameters, only modest maneuvers would be required with no need for spin recovery.

B-52s prior to the G models had very small ailerons with a short span that was approximately equal to their chord. These "feeler ailerons" were used to provide feedback forces to the pilot's control yoke and to fine tune the roll axes during delicate maneuvers such as aerial refueling.[102] Due to twisting of the thin main wing, conventional outboard flap type ailerons would lose authority and therefore could not be used. In other words, aileron activation would cause the wing to twist, undermining roll control. Six spoilerons on each wing are responsible for the majority of roll control. The late B-52G models eliminated the ailerons altogether and added an extra spoileron to each wing.[102] Partly because of the lack of ailerons, the B-52G and H models were more susceptible to Dutch roll.

Present day high subsonic passenger jets mostly have two sets of ailerons, inboard and outboard, plus spoilerons. Inboard ailerons to be used at high speeds, also to eliminate aileron reversion.

"more susceptible to Dutch Roll".  Show me an airliner that can do this.  I wonder how much the outboard engine pods and fuel tanks helped here.

@AdityaSharma Indeed, have updated.

Sure, also really appreciate mentioning firsé ailerons as a solution. Good day!

in this configuration when aileron is up additional drag is created because of split.

Looking at the following picture taken from this NACA report, it can be seen that, the lift coefficient being the same, a split flap/aileron generates less drag than a plain flap/aileron, especially at high AoA:

So, with a split flap/aileron the opposite of what you'd like to achieve actually happens.
Also for the pitching moment similar results are obtained, with the split flap/aileron modifying the moment less than a plain flap/aileron, the $C_l$ being the same.
Both plain and split flaps/ailerons were superseded after WW2 by the more efficient slotted flaps/ailerons.

Bonus material
Spoilers are normally used to... well spoil the airflow on the upper surface of a wing, basically triggering a controlled stall.
Even if they can be termed spoileron and used as primary control surfaces (typical example is the B-52 and many, if not all, jet fighters), they shouldn't be confused with ailerons as such:

aileron are deployed in a more or less asymmetrical way between left and right wing (one goes up and the other goes down); spoilers can only go up, locally reducing lift and increasing drag.
ailerons end at the trailing edge of the wing and extend for some 0.2 chord; to be effective, spoilers are normally placed at a distance of some 0.6 chord from the leading edge and do not extend to the end of the wing.
on the B-52 spoilers were preferred to ailerons due to aeroelastic problematics of the very elongated wing.

The B52 has "spoilerons".  Roll control is possible by raising spoilers differentially.  The advantage is that adverse yaw is not created.
On the B-52, this greatly relieved stresses on its very tall vertical stabilizer of early models, which was needed for stability at the 50,000+ feet and high subsonic airspeeds the swept wing bomber was designed to fly at after design modifications in the late 1940s, by eliminating the need for rudder inputs to counter-act adverse yaw.
It works so well, later models have no ailerons at all.  Spoilerons also help reduce wing twisting forces that ailerons can produce at higher airspeeds.

Comments are not for extended discussion; this conversation has been moved to chat.

First, we can define split flaps from the reference and note:  there is not a picture of a split flap in the question.
What is pictured in the top image is more akin to a spoileron.
The middle image shows an airfoil with no control surface deflection, and the bottom one shows a downward flap deflection.
It is the downward deflection that causes adverse yaw.  Differential spoilerons will cause a roll into the turn, rather than away, eliminating the need for rudder to counter-act adverse yaw.